1. Field of the Invention
The present invention generally relates to drive control units, drive control methods and image forming apparatuses, and more particularly to a drive control unit and a drive control method for controlling speeds of a plurality of driving parts, and an image forming apparatus which employs such a drive control unit or drive control method.
2. Description of the Related Art
Conventionally, as methods of forming color images, there are the intermediate transfer method and the tandem method. The intermediate transfer method forms a toner image on one photoconductive drum, one color at a time, and obtains the full color image by successively transferring the toner images of different colors on a transfer member. The tandem method arranged a plurality of photoconductive drums in parallel, forms the toner image of one color at each photoconductive drum, and obtains the full color image by successively transferring the toner images of different colors on the transfer member which successively passes the plurality of photoconductive drums. According to the tandem method, it is possible to carry out a high-speed image formation by operating the plurality of photoconductive drums approximately at the same time in synchronism with each other.
Since the tandem method overlaps the toner images of the different colors, the plurality of photoconductive drums must rotate without flutter and in accurate synchronism with each other. Hence, in the tandem method which uses the plurality of photoconductive drums (for example, four photoconductive drums in order to obtain the full color image), the rotational speeds of the photoconductive drums are controlled independently so as to match the toner images of different colors that are overlapped.
The tandem method can form the full color image without registration error of each of the colors (for example, black, yellow, magenta and cyan) if the rotational speeds of the plurality of photoconductive drums are all constant. However, since the plurality of photoconductive drums are controlled independently, it is difficult to perfectly match the rotational speeds of the plurality of photoconductive drums.
In other words, in the case of the tandem method, even if a target rotational speed is set with respect to each of the plurality of photoconductive drums, each photoconductive drum undergoes a fluctuation in its rotational speed that is peculiar to each photoconductive drum, due to eccentricity of a drum shaft and mounting error of a driving part and the photoconductive drum itself. As a result, the registration error in which the overlapping toner images of the different colors do not match on the transfer member is caused by the fluctuations in the rotational speeds of the plurality of photoconductive drums.
FIG. 1 is a system block diagram showing an example of a drive control unit for the photoconductive drums. The drive control unit shown in FIG. 1 includes drive control systems for each of the colors, which are black, yellow, magenta and cyan in this particular case. The drive control system for black includes a control part 1K, a motor 2K, a photoconductive drum 3K and an encoder 4K.
The motor 2K is coupled to the photoconductive drum 3K via a driving shaft and drives the photoconductive drum 3K. The encoder 4K detects the actual rotational speed of the motor 2K. In addition, the control part 1K controls the rotational speed of the motor 2K depending on a difference between a target rotational speed which is a constant value and the actual rotational speed that is fed back from the encoder 4K.
The drive control system for yellow includes a control part 1Y, a motor 2Y, a photoconductive drum 3Y and an encoder 4Y. The drive control system for magenta includes a control part 1M, a motor 2M, a photoconductive drum 3M and an encoder 4M. The drive control system for cyan includes a control part 1C, a motor 2C, a photoconductive drum 3C and an encoder 4C. The drive control systems for yellow, magenta and cyan operate similarly to the drive control system for black described above.
However, even though the drive control unit having the structure shown in FIG. 1 controls the rotational speed of the motor 2K depending on the difference between the constant target rotational speed and the actual rotational speed that are fed back from the encoder 4K, the photoconductive drum 3K undergoes a fluctuation in its rotational speed that is peculiar to the photoconductive drum 3K, due to the eccentricity of the drum shaft and the mounting error of the driving part and the photoconductive drum 3K itself. As a result, the registration error in which the overlapping toner images of the different colors do not match on the transfer member is caused by the fluctuations in the rotational speed of the photoconductive drum 3K.
In addition, in the drive control systems for yellow, magenta and cyan, the registration error is generated similarly to the drive control system for black. In other words, the rotational speeds for one revolution of the photoconductive drums 3K, 3Y, 3M and 3C fluctuate as shown in FIG. 2. FIG. 2 is a graph showing the actual speed fluctuations for one revolution of the photoconductive drums 3K, 3Y, 3M and 3C, wherein v denotes the rotational speed in arbitrary units and t denotes the time in arbitrary units. In the following description, the fluctuation of the rotational speed for one revolution of each of the photoconductive drums 3K, 3Y, 3M and 3C will be referred to as a speed fluctuation profile.
A Japanese Laid-Open Patent Application No. 2002-72816 proposes a process of controlling the rotational speed of each photoconductive drum to approach the target rotational speed, by detecting the actual speed fluctuation profile of each photoconductive drum and adding data having an inverted phase (that is, a phase shifted by 180 degrees) with respect to the speed fluctuation profile, so as to correct the rotational speed of each photoconductive drum.
However, according to the process proposed in the Japanese Laid-Open Patent Application No. 2002-72816, the rotational speeds of the plurality of photoconductive drums are independently controlled, similarly to the conventional method. For this reason, even if the rotational speeds of the photoconductive drums are controlled to approximate the target rotational speed, it is difficult to completely eliminate the fluctuation in the rotational speeds of the photoconductive drums and to perfectly match the rotational speeds of each of the photoconductive drums.